Alzheimer's disease, also referred to as Alzheimer's dementia or AD is a progressive neurodegenerative disorder that causes memory loss and serious mental deterioration. Diagnosticians have long sought a means to definitively identify AD during the lifetime of demented patients, as opposed to histopathological examination of brain tissue, which is the only present means available for rendering an ultimate diagnosis of AD. AD is the most common form of dementia, accounting for more than half of all dementias and affecting as many as 4 million Americans and nearly 15 million people worldwide. Dementia may start with slight memory loss and confusion, but advances with time reaching severe impairment of intellectual and social abilities. At age 65, the community prevalence of AD is between 1–2%. By age 75, the figure rises to 7%, and by age 85 it is 18%. The prevalence of dementia in all individuals over age 65 is 8%. Of those residing in institutions, the prevalence is about 50%, at any age.
The social impact of this disease is enormous, caused by the burden placed on caregivers, particularly in the latter stages of the disease. The substantial economic costs are largely related to supportive care and institutional admission. The rapidly increasing proportion of elderly people in society means that the number of individuals affected with AD will grow dramatically, therefore finding an early accurate diagnosis and a cure for AD is becoming an issue of major importance world wide.
When an individual is suspected of AD, several recommended tests are performed : (1) Mini Mental State Examination (MMSE)—an office-based psychometric test in the of a Functional Assessment Questionnaire (FAQ) to examine the scale for functional autonomy, (2) Laboratory tests—complete blood count, measurement of thyroid stimulating hormone, serum electrolytes, serum calcium and serum glucose levels, (3) Neuroimaging—most commonly used is computed tomography (CT) which has a role in detecting certain causes of dementia such as vascular dementia (VaD), tumor, normal pressure hydrocephalus or subdural hematoma. However, neuroimaging is less effective in distinguishing AD or other cortical dementias from normal aging. In primary care settings, some suggest that CT could be limited to atypical cases, but others recommend routine scanning. Magnetic resonance imaging (MRI) currently offers no advantage over CT in most cases of dementia.
While Alzheimer's is the most common form of dementia, accounting for at least 60% of cases, diagnostic procedures for determining the exact cause of dementia, among more than 80 different species, is difficult at best. Furthermore, the currently performed tests are inadequate in differentiating AD from other types of dementia.
In comparison to other disease areas, the field of dementia raises questions concerning the value of diagnosis, since there is currently no cure or effective therapy available. In dementia, as in all other branches of medicine, the certainty of a diagnosis has an important impact on the management of the patient. While AD cannot be cured at present time, there is symptomatic treatment available and the first drugs (acetylcholinesterase inhibitors) for the temporary improvement of cognition and behavior are now licensed by the U.S. Food and Drug Administration. Other drugs are at different stages of clinical trials: (1) Drugs to prevent decline in AD—DESFERRIOXAMINE, ALCAR, anti-inflammatory drugs, antioxidants, estrogen, (2) Neurotrophic Factors: NGF, (3) Vaccine: the recent most exciting report by Schenk et al. (Nature 1999;400:173–7) raises the hope of a vaccine for AD.
The specificity of the various therapies thus require sophisticated diagnostic methodologies, having a high degree of sensitivity for AD, in order to insure their success.
Currently there are a multitude of tests available which aid in the diagnosis of AD. However, the only true existing diagnosis is made by pathologic examination of postmortem brain tissue in conjunction with a clinical history of dementia. This diagnosis is based on the presence in brain tissue of neurofibrillary tangles and of neuritic (senile) plaques, which have been correlated with clinical dementia. Neuritic plaques are made up of a normally harmless protein called amyloid-beta. Before neurons begin to die and symptoms develop, plaque deposits form between neurons early on in the disease process. The neurofibrillary tangles are interneuronal aggregates composed of normal and paired helical filaments and presumably consist of several different proteins. The internal support structure for brain neurons depends on the normal functioning of a protein called tau. In Alzheimer's disease, threads of tau protein undergo alterations that cause them to become twisted. The neurohistopathologic identification and counting of neuritic plaques and neurofibrillary tangles requires staining and microscopic examination of several brain sections. However, the results of this methodology can widely vary and is time-consuming and labor-intensive.
Given the ability of both current and prospective pharmacological therapies to forestall and/or reverse the onset and/or progress of Alzheimer's dementia, an early diagnosis of AD will assist to better manage the care of patients. There are many cases where non-AD dementia could be confused with AD dementia. Such examples include small, undetected strokes which temporarily interrupt blood flow to the brain. Clinically depressed patients or those with Parkinson's disease can also experience lapses in memory. Many older people are on a variety of medications which as a side effect may, alone or in conjunction, impair their ability to perform cognitive tasks.
Thus, if diagnostic techniques for the early differentiation of AD could be provided, physician's would achieve an enhanced ability to prescribe appropriate therapeutic intervention at an early stage in the pathogenesis of this disease.
Various biochemical markers for AD are known and analytical techniques for the determination of such markers have been described in the art. As used herein the term “marker” “biochemical marker” or “marker protein” refers to any enzyme, protein, polypeptide, peptide, isomeric form thereof, immunologically detectable fragments thereof, or other molecule that is released from the brain during the course of AD pathogenesis. Such markers include, but are not limited to, any unique proteins or isoforms thereof that are particularly associated with the brain.
Glutamine synthetase (GS) is recognized as an astrocyte-specific enzyme involved in the regulation of ammonia and glutamate metabolism that is over-expressed following brain injury (Norenberg and Martinex-Hernandez, Brain Res 1979;161:303). A few studies on the clinical role of glutamine synthetase have been reported: Gunnersen and Haley (Proc Natl Acad Sci USA 1992;89:11949) found monomeric GS protein in 38 of 39 AD cerebrospinal fluid (CSF) samples, Tumani et al. (Arch Neurol 1999;56(10):1241) describe that the concentration of GS in lumbar CSF of patients with AD is increased significantly but nonspecifically (i.e. also increased in VaD, schizophrenia and ALS). On p.1244, the left-hand column, Tumani states that GS was not found in serum.
Neuron-specific gamma-enolase (NSEγγ) and S1OOB proteins, abundant in the brain, are also useful markers for assessing the extent of brain damage: NSEγγ for neuronal damage and S100B for astrocyte damage. Concentrations of NSE and S100B proteins from cerebrocortical regions have been examined by means of enzyme linked immunosorbent assay (ELISA). The levels of these proteins in frontal cortex of AD patients were found to be significantly elevated (Kato et al. J Mol Neurosci, 1991;3(2):95). Activated astrocytes over-expressing S100B have been intimately associated with the neuritic β-amyloid plaques of AD (Sheng et al. J Neurosci Res, 1994;39:398, Mrak et al. J Neuropathol Exp Neurol 1996;55:273).
There are a number of different potential uses for biomarkers in AD evaluation, and each use could involve a different marker or set of markers. Such uses may include, but are not limited to, the use of a marker to distinguish AD from other causes of dementia; distinguishing dementia from the non-pathological effects of aging; monitoring the progress of the disease after clinical symptoms become apparent; utilization of a surrogate to monitor the efficacy of the forthcoming therapies for AD; and isolating markers which have utility as risk assessment factors for AD; and identifying both the earliest biological changes occurring in the brain and other changes that occur as the disease progresses. Ideally, it would be preferable to isolate a single marker to fulfill all requirements with a high degree of sensitivity and specificity, however this may be an unreasonable goal. Any individual marker needs to be assessed by sensitivity, specificity, reliability and validity for the type of clinical situation to which it is meant to apply. A marker which is poor at distinguishing AD from other causes of dementia, could nevertheless be an excellent marker for monitoring the progression of the disease process or the response to therapy.
With regard to diagnostic devices, the clinical evaluation and use of point-of-care tests utilizing biological markers are valuable tools for evaluating risk, monitoring disease progression and guiding therapeutic interventions. The advantages which flow from the use of biological markers as diagnostic tools include strengthening the certainty of the clinical diagnosis, distinguishing AD from other causes of dementia, and quantifying the severity of the disease and rate of progression. In addition, tests using biological markers should be rapid, non-invasive, simple to perform and inexpensive.
What is lacking in the art is a relatively non-invasive method and device therefore effective for definitively diagnosing Alzheimer's dementia in living patients. Additionally, a definitive method of assessing the risk of developing AD is greatly needed.